The present invention relates to an image pickup apparatus, and more particularly to an image pickup apparatus that includes a focus detecting apparatus that detects a focusing status of an image pickup lens. The present invention is suitable, for example, for a digital still camera.
Along with widespread digital and video cameras, etc., the image pickup apparatuses have been increasingly required for the high quality and improved operability. In particular, a focus detecting apparatus or optical system (while these terms are interchangeable in this application) which detects a focusing status of an image pickup lens is required for the improved precision and speed. For instance, the recent mainstream is a focus detecting apparatus of a phase difference detection (“PDD”) system that needs a much shorter focusing time period than the contrast detection system.
Referring to FIG. 19, a description will be given of the PDD focus detection principle. FIG. 19 is a schematic sectional view of a PDD focus detecting optical system 1000. A field lens 1010 images an exit pupil of an image pickup lens IL on pupil planes of at least a pair of secondary imaging lenses 1020. Thereby, the light incident upon the secondary imaging lenses 1020 are emitted from regions that have the same areas and are located at different positions, on the exit pupil of the image pickup lens IL. A stop 1030 having openings corresponding to the secondary imaging lenses 1020 is located in front of the secondary imaging lenses 1020. The secondary imaging lenses 1020 re-image an aerial image which the image pickup lens IL has imaged near the field lens 1010, onto at least a pair of sensors (photoelectric conversion elements) 1040. Since re-imaged positions relatively vary based on an imaging position of the photographing optical axis of the aerial image, this configuration can detect a focusing status of the image pickup lens IL (i.e., a defocus amount and a defocus direction) by detecting a variation amount of the relative position and moving direction.
There are proposed a multipoint focus detection that provides multiple focus detecting regions used to detect longitudinal and lateral lines in the photographing screen, an area type focus detection that detects focus in a continuous area in a wide range, etc. See Japanese Patent Application, Publication No. 9-184965. An image pickup apparatus disclosed in this reference basically detects a focusing status of the image pickup lens by utilizing the PDD system. In this reference, a subsidiary mirror that introduces the light that has passed the image pickup lens to a focus detecting optical system maintains an imaging relationship between an exit pupil of the image pickup lens and a stop of the focus detecting optical system, and serves as a field lens. In other words, the subsidiary mirror and the stop serve as pupil splitting means, and can introduce plural lights that have passed the exit pupil of the image pickup lens to the focus detecting optical system.
FIG. 20 is a schematic plane view showing a stop 1100 and a secondary imaging lens 1200 in the focus detecting optical system in the above reference. The stop 1100 has a pair of openings 1110 and 1120 and a pair of openings 1130 and 1140. The secondary imaging lens 1200 includes a pair of lens elements 1210 and 1220 and a pair of lens elements 1230 and 1240, corresponding to two pairs of openings. The secondary imaging lens 1200 having four lens elements images the lights perpendicularly split by the openings 1110 and 1120 and the lights horizontally split by the opening 1130 and 1140 among the lights that have passed the exit pupil of the image pickup lens, thereby forming four optical images on a sensor or secondary imaging plane. The focusing status is recognized by detecting phase shifts of the four optical images caused by the defocus of the image pickup lens.
FIG. 21 is a schematic plane view showing a sensor 1300 in the focus detecting optical system in the above reference. The sensor 1300 has four photoelectric conversion areas for the secondary imaging lens 1200. The lens elements 1210 and 1220 correspond to the photoelectric conversion areas 1310 and 1320, respectively. The lens elements 1230 and 1240 correspond to the photoelectric conversion areas 1330 and 1340, respectively. Optical images projected onto the photoelectric conversion areas 1310 and 1320 result from the lights that have passed the openings 1110 and 1120 or the perpendicularly split light of the exit pupil of the image pickup lens. As the image pickup lens defocuses, these optical images move in an approximately perpendicular direction. Therefore, the photoelectric conversion areas 1310 and 1320 each have plural closely arranged, perpendicularly extending line sensors. Similarly, the photoelectric conversion areas 1330 and 1340 each have plural closely arranged, horizontally extending line sensors.
FIG. 22 is a schematic plane view of an optical image OI in each photoelectric conversion area of the sensor 1300. Optical images OI1 and OI2 correspond to the photoelectric conversion areas 1310 and 1320, and optical images OI3 and OI4 correspond to the photoelectric conversion areas 1330 and 1340. Each optical image on a prospective imaging plane or an image sensor plane in the image pickup apparatus has a rectangular lattice shape projected on the sensor 1300, but its shape is distorted with respect to a centerline C as a symmetrical axis due to a distortion of the subsidiary mirror etc. In other words, the optical image is distorted into a sectorial shape.
FIG. 23 is a schematic plane view of a prospective imaging plane PIS of the image pickup apparatus, showing a back-projected state of the photoelectric conversion area 1300 shown in FIG. 21. As discussed above, since the rectangular lattice shape on the prospective imaging plane PIS has a sectorial shape on the sensor 1300, the photoelectric conversion area that would be otherwise rectangular on the secondary imaging plane has a sectorial distortion on the prospective imaging plane PIS. Hence, a back-projected image RP1 of the photoelectric conversion areas OI1 and OI2 has a sectorial shape. Since the photoelectric conversion areas OI1 and OI2 have approximately the same shape on the prospective imaging plane PIS, FIG. 23 shows only one back-projected image RP1. Similarly, a back-projected image RP2 of the photoelectric conversion area OI3 and OI4 has a sectorial shape.
The above reference arranges approximately perpendicular line sensors shown by the back-projected image RP1 in a wide range, and approximately horizontal line sensors shown by the back-projected image RP2 at the center. The perpendicular line sensors detect the focusing status of the subject image having a contrast component in the approximately perpendicular direction, and the horizontal line sensors detect the focusing status of the subject image having a contrast component in the approximately horizontal direction: The back-projected image RP1 provides lateral line focus detection, the back-projected image RP2 provides vertical line focus detection, and a so-called cross type focus detection is provided in the common area.
The focusing status is detected, for example, by using the optical images OI1 and OI2 to detect defocus caused perpendicular movements. However, strictly speaking, these optical images have no similarity since the symmetrical axis of the optical image is the centerline C. The similarity especially lowers as a distance from the center of each optical image increases. Thus, the rectangular photoelectric conversion area including a one-dimensionally arranged line sensors has an error in the focus detection result. This is true of the optical images OI3 and OI4.
One proposed solution is a focus detecting apparatus that includes a light blocking mask having an opening shape corresponding to an optical image's distortion in the photoelectric conversion area, and eliminates an error in the focus detection. See, for example, Japanese Patent Application, Publication No. 61-15112. Referring now to FIG. 24, an illustrative description will be given of most distant line sensors 1312 and 1322 from the center of the respective optical images in a pair of photoelectric conversion areas 1310 and 1320.
FIG. 25 is a schematic plane view of the prospective imaging plane PIS of the image pickup apparatus, showing a back-projected image RP1′ of the line sensor 1312 and a back-projected image RP2′ of the line sensor 1322. Strictly speaking, as described above, the back-projected images RP1′ and RP2′ correspond to each other due to the optical images' distortions, although FIG. 25 exaggerates the shift. When the pair of line sensors do not correspond to each other, the defocus occurs for the subject. Thus, the light blocking mask is provided which provides a light receiving area or photoelectric conversion area only at the common area of the pair of line sensors.
FIG. 26 is an enlarged view of the back-projected images RP1′ and RP2′ shown in FIG. 25, and shows a light blocking mask SM that blocks the light other than the common area between the line sensors 1312 and 1322. Therefore, the pair of line sensors receive the light of the common part of the subject, and can eliminate an error in the focus detection for the subject. Although the two line sensors do not correspond to each other in the approximately longitudinal direction, the error can be eliminated, for example, by a signal processing correction disclosed in Japanese Patent Application, Publication No. 10-311945.
As the image pickup lens defocuses, the back-projected images RP1′ and RP2′ (line sensors 1312 and 1322 move in the approximately longitudinal direction. Nevertheless, strictly speaking, the distortion of the focus detecting optical system varies the moving directions according to an image height, and deforms the back-projected images RP1′ and RP2′ of the line sensors 1312 and 1322. When the light blocking mask provides is a common light receiving area between the line sensors 1312 and 1322 during an in-focus time period, an offset occurs between these two line sensors' light receiving areas during a defocus or out-of-focus time period and this offset increases as the defocus amount becomes large.
It is important for the PDD system to receive the light at the common area of the pair of line sensors on the prospective imaging plane during an in-focus time period, and to receive the light at a position that shifts only in a one-dimensional direction as a pupil splitting direction during a defocus time period. In other words, it is important that no parallaxes occur in a direction other than in the one-dimension direction.
Nevertheless, the above prior art cause a parallax in a direction other than the one-dimensional direction, and thus an error in focus detection for the subject during a defocus time period. Thereby, the image pickup lens cannot be in focus in one focus detection action, and needs plural focus detection actions, deteriorating quick in-focus of the image pickup lens or shortened detection time period, which is the most favorable advantage of the PDD system.
The above error occurs among all the line sensors in the sensor in the focus detecting optical system, and is particularly conspicuous in a distant line sensor from the center of the optical image or in an attempt to expand the focus detection range from the conventional range.